KR20180059716A - Cast steel with excellent flexural strength and tool and die using the same - Google Patents

Abstract

The present invention relates to various metal molds and other tools used for forming parts of metal materials and components thereof. More specifically, the present invention is characterized in which components made through casting of alloy tool steel are applied to the components of the metal mold and the tool. In addition, the present invention is characterized in which a microstructure is controlled through addition of an appropriate amount of Ca during the casting to improve bending strength of the alloy tool steel after hardening heat treatment.

Description

Technical Field [0001] The present invention relates to a cast steel having excellent bending strength and a mold and a tool using the cast steel.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various molds and other tools used for forming parts of metal materials, and more particularly, to a steel part having excellent bending strength and components such as molds and tools using the same.

Alloy tool steels contain a large amount of carbon and other alloying elements compared to steels for general structure, so that high hardness and wear resistance can be obtained. Therefore, alloy tool steels are applied to major parts of various molds and tools.

Alloy tool steels generally are finished as final products through the following process.

(1) Casting: Making cast ingot of specific composition.

(2) Hot working (forging, rolling, etc.): Minimization of casting defects such as shrinkage holes and segregation, and shaping of intermediate products (plate, sheet, bar, etc.).

(3) Spheroidalizing heat treatment: Heat treatment to facilitate cutting of intermediate product.

(4) Primary shaping (rough milling): Cutting the shape of molds and tool parts.

(5) Hardening heat treatment: Quenching and tempering heat treatment to control various physical properties such as hardness of first machined parts.

(6) Second shape machining (finishing): Precision machining to the final dimensions and shape of the parts of the tool and the tool.

However, the alloy tool steel is basically a material having high cutting resistance, and it takes much time and expense to process the primary shape among the above processes. Therefore, instead of cutting the wrought alloy as described above, it may be considered to form the primary shape of the mold and the tool part directly in the casting process. In this case, (2) the hot forging process is omitted, (3) the spheroidizing heat treatment process, and (4) the cost reduction by omitting or minimizing the primary shaping process is possible. Particularly, (4) Minimization of the primary shape machining process, that is, minimization of the cutting amount, not only reduces the process cost, but also reduces the loss of the raw material, thereby achieving high cost saving.

In addition, in the case of forged and rolled materials supplied in a limited size and shape, the number of additional processes such as joining and assembly after individual processing increases in a large number of parts in the construction of a large mold and a tool, It is possible to have a higher degree of freedom with respect to the size and shape of the mold and the tool. Therefore, it has many merits such as reduction of material and processing cost, shortening delivery time by shortening process and material distribution process, and flexible response to product design and design change.

However, in the same alloy composition, casting materials are inferior in durability to forged and rolled materials due to high defect density, and therefore, they are limited to applications in severe molding conditions as in the case of forming high strength metal materials. Cast steel having a composition based on AISI-D2 (KS-STD11) which is a representative cold metal steel has conventionally been applied to a metal mold for cold press forming (Patent Document 1).

However, molds and tool parts made of cast steel described in Patent Document 1 have a durability that is insufficient to be applied to the molding of high-strength metal parts in recent years. Therefore, the applied amount thereof is gradually decreased and replaced with a forged and rolled material having low economic efficiency have. This leads to an increase in the manufacturing cost of the mold, leading to a reduction in the price competitiveness of the manufacturing industry such as automobiles, electronics, and household appliances producing the products by using the corresponding molds.

Patent Document 1: Japanese Patent Application Laid-Open No. 10-2002-0091375 (published Dec. 2002, 2002)

An object of the present invention is to provide a cast steel capable of solving the conventional problems in order to improve the economic efficiency of the production of a mold and a tool.

Another object of the present invention is to provide a mold or a tool to which the cast steel is applied to a component.

In order to attain the above object, the cast steel according to the present invention comprises, by weight, C: more than 0.9% to less than 1.0%, more than 5.0 to less than 5.3% of Cr, more than 1.2 to less than 1.3% Or more and less than 0.5%, Si: not less than 0.4% and not more than 0.5%, Mn: not less than 0.4% and not more than 0.5%, Ni: not less than 0.2% and not more than 0.25% Wherein the Ca content is 0.02 to 0.2% by weight in the raw material before casting, and the balance of Fe and unavoidable impurities.

At this time, it is more preferable that the Ca addition amount is 0.05 to 0.1% by weight in the raw material before casting.

Also, the cast steel may have an O content of 120 ppm or less.

In order to achieve the above object, a mold or a tool according to an embodiment of the present invention is characterized in that the composition comprises, by weight, C: more than 0.9% to less than 1.0%, Cr: more than 5.0% to less than 5.3% At least 0.4% to less than 0.5%, at least 0.4% to less than 0.5% of Si, at least 0.4% to less than 0.5% of Si, at least 0.4% The Ca-containing compound is further included, and Ca is added in an amount of 0.02 to 0.2% by weight in the raw material before casting, and is characterized by being a cast steel made of the remaining Fe and unavoidable impurities.

The bending strength of the cast steel according to the present invention having the above alloy composition can be greatly improved by 90% or more than that of the existing cast steel through control of alloy components, particularly Ca content.

Accordingly, the cast steel according to the present invention can have excellent durability in casting, and can be applied to components of a mold and a tool.

Fig. 1 shows the change of the bending strength according to the Ca addition amount.
Fig. 2 shows the microstructure of Comparative Example 1 in which Ca was not added, and Fig. 2 (b) shows the microstructure of Inventive Example 2 in which Ca addition ratio was 0.05 wt% .
Fig. 3 shows changes in oxygen content in the examples by Ca addition.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a cast steel having excellent bending strength according to the present invention and a mold and a tool using the same will be described in detail with reference to the accompanying drawings.

Cast steel

The cast steel according to the present invention contains, by weight%, C: more than 0.9% to less than 1.0%, Cr: more than 5.0% to less than 5.3%, Mo: more than 1.2% to less than 1.3%, V: Si: 0.4 to 0.5%, Mn: 0.4 to 0.5%, Ni: 0.2 to less than 0.25%. Further, the cast steel according to the present invention may further contain Ca or a Ca-containing compound, wherein the Ca content is 0.02 to 0.2% by weight in the raw material before casting. The rest of the alloy components are composed of Fe and unavoidable impurities.

Hereinafter, the role and content of each component contained in the cast steel according to the present invention will be described.

[C: more than 0.9% and not more than 1.0% by weight]

Carbon (C) forms carbides in the microstructure, some of which are dissolved in the matrix to form martensite and increase the strength. Therefore, it imparts hardness, abrasion resistance and hardenability to the material, but generally causes excessive embrittlement. A content exceeding 0.9 wt% is required for a sufficient strengthening effect, but a content lower than 1.0 wt% shows a lower bending strength due to embrittlement as in Comparative Example 1.

[Cr: more than 5.0% and less than 5.3%]

Chromium (Cr) is one of the main forming elements of carbide. It forms a variety of carbides by bonding with C, thereby giving hardness and abrasion resistance to the material, giving hardenability and contributing to improvement of corrosion resistance. In the alloy tool steel, the carbides formed by the element are mainly M ₇ C ₃ and M ₂₃ C ₆ , and the content of more than 5.0 wt% is required for sufficient carbide formation. However, when it is added in excess, And thus it is limited to less than 5.3% by weight.

[Mo: more than 1.2% and less than 1.3%]

Molybdenum (Mo) is solved in carbides to improve the physical properties of carbides, and particularly contributes to improvement of tempering resistance which suppresses hardness decrease during tempering treatment. In particular, it is an important element supplementing the strengthening effect which is insufficient in Comparative Examples 2-4 and 1-3, in which the contents of C and Cr, which are strengthening elements, are lower than those of Comparative Example 1. [ However, it is a strong element of segregation after casting in contrast to its excellent effect, and it is also an element causing unevenness of the material and central defect of the cast material.

Accordingly, in consideration of the relatively low C and Cr contents, the content of Mo is more than 1.2% by weight, but the Mo content is less than 1.3% by weight considering the negative effect of segregation and Mo as an expensive element.

[V: 0.4% or more and less than 0.5%]

Vanadium (V) enhances abrasion resistance by promoting the formation of primary carbides formed during the casting process. It enhances hardness and wear resistance by forming very fine VC carbide during tempering at a temperature of 500 ° C or higher and strengthening the matrix. Contributing. Therefore, as in the examples other than Comparative Example 1, a steel having a low C and Cr content requires a V addition amount of 0.4 wt% or more for the purpose of supplementing the strengthening effect. However, since the increase of the V content causes a problem of embrittlement due to the coarsening of the carbide, the V content is limited to less than 0.5 wt% in the present invention.

[Si: 0.4% or more and 0.5% or less]

Silicon (Si) has a large effect of strengthening the solid solution and contributes to improvement of machinability. In addition, Si, together with Mo and W, can also contribute to improvement of hardenability and softening resistance. Therefore, Si is preferably added in an amount of not less than 0.4% by weight, but excessive amounts of the Si addition result in brittleness and excessive formation of retained austenite, resulting in unstable structure and shape. Therefore, in the present invention, the content of Si is limited to 0.5% by weight or less.

[Mn: 0.4% or more and 0.5% or less]

Manganese (Mn) has a positive effect of addition of 0.4 wt% or more, because it can enhance the solubility and hardenability at low cost. However, if the Mn content exceeds 0.5 wt%, the surface quality and toughness after high temperature maintenance are lowered, and a large amount of austenite is left after quenching, resulting in lower hardness and heat treatment and change in shape during operation of the mold.

[Ni: not less than 0.2% and not more than 0.25%]

Nickel (Ni) has an effect of increasing the toughness of steel products, so that addition of Ni of 0.2 wt% or more can contribute to improvement of durability of cast steel. On the other hand, in the metal mold, the hardness can be lowered due to an increase in retained austenite, and the upper limit is limited to less than 0.25 wt%.

[Ca: 0.02% or more and 0.2% or less]

Ca plays an important role in improving bending strength through microstructural and microstructural effects of MnS inclusion solidification and deoxidation. Such Ca can be added to Ca itself or in the form of a compound with other elements such as Ca-Si.

At this time, the Ca content is preferably 0.02 to 0.2% by weight in the raw material before casting. Here, the reason why the Ca content is determined as the content of the raw material before the casting is that, in the case of Ca, it is considered that the Ca content is partially reduced in the final product state after bonding with O. The effect of improving the bending strength by deoxidation or the like becomes remarkable from a Ca content of 0.02 wt% or more. However, when the Ca content exceeds 0.2% by weight, further deoxidation is not performed, and the desired bending strength may be lowered. Referring to FIG. 1, the Ca content is more preferably 0.05 to 0.1% by weight in the raw material before casting.

On the other hand, tungsten (W) has an effect similar to that of Mo, but the relative addition effect is known to be higher than Mo. That is, W can have a better reinforcing effect. However, in the case of W, since the negative effect due to segregation is further strengthened, W is not intentionally added other than impurities in the present invention.

Cast steel  Manufacturing method

The cast steel according to the present invention can be manufactured by a casting process as described below, and the process conditions can be changed as needed.

For this purpose, a raw material for satisfying the above composition is melted at a high temperature to form a molten metal. The Ca- or Ca-containing compound is added in advance to the raw material or added during the formation of the molten metal so that the weight ratio of Ca to the raw material can be satisfied regardless of the kind of the additive to be used.

The primary shape of the product is realized by injecting the molten metal into the mold (sand mold, ceramic mold, etc.) close to the final shape of the mold and tool components and solidifying the product. After completion of solidification, It is also possible to perform additional cutting processing for bringing it closer. In this case, spheroidizing heat treatment may be performed in order to reduce the cutting resistance. The heat treatment is a process of cooling the product to room temperature at a slower rate than the case where the product is maintained in a temperature range of 700 ° C to 900 ° C and then cooled in the air.

Thereafter, the product is subjected to a hardening heat treatment, which consists of quenching and tempering. The quenching is a process of maintaining the product at a temperature of 900 ° C to 1100 ° C, followed by cooling to a temperature of 600 ° C or lower, and a tempering process of maintaining the quenched product at a temperature of 150 ° C to 600 ° C and then cooling to room temperature. The above tempering heat treatment can be repeated two or more times to control the physical properties of the product.

In the heat treatment processes, the high-temperature holding time of the product depends on the size of the product. Generally, it is set to 1 hour per 1 inch (25.4 mm), but the shape of the product, the kind of heat treatment furnace, The manufacturer may decide according to the situation of This is also true for the temperature and cooling rate, and there is no particular restriction in the line that satisfies the properties required for the final product.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

Table 1 shows the alloy composition in this example and the addition ratio of Ca in the raw material, and shows quenching at 1030 ° C and bending strength after tempering at 520 ° C.

A vacuum heat treatment furnace was used for quenching and tempering, and the specimens were heated at a rate of about 5 ° C / min and cooled at a similar rate. During the quenching, the high temperature retention time was 30 minutes, while the tempering heat treatment was 120 minutes, and the same tempering treatment was repeated twice. The bending strength was measured by a four-point bending test according to ASTM C1161 or KS L 1591.

Table 1 shows the composition of Example (mass ratio,%), Ca addition ratio and bending strength, and oxygen content (mass ratio, ppm).

[Table 1] (Unit: wt%, based on raw material for Ca)

Comparative Example 1 shows that the bending strength is improved by 90% or more through application of an alloy composition corresponding to Comparative Example 2-4 and Inventive Example 1-3 as a conventional cast steel for a cold mold. In Comparative Examples 2-4 and 1-3, the effect of the Ca additive can be confirmed, which is more clearly confirmed in FIG.

By the addition of Ca, a noticeable improvement in bending strength can be confirmed in Inventive Examples 1-3. Referring to FIG. 1, the bending strength was higher than that of Examples 2 to 3 in which the Ca content was 0.05 to 0.1 wt%. However, it can be seen that the bending strength decreases when the addition amount of Ca exceeds 0.1%. As can be seen from Fig. 2, the enhancement of the bending strength by the addition of Ca can be seen to be primarily due to the miniaturization of the structure.

On the other hand, Ca is considered to have contributed to the improvement of the physical properties of the cast material because Ca is inevitably detected in most of the steel materials and spheroidizing the MnS inclusions and also acts as a deoxidizer.

The deoxidation effect by Ca addition can be confirmed in FIG. 3, and it can be seen that the deoxidation effect is maximized with the use of Example 2. Compared with Comparative Example 2 in which Ca was not added, the content of O in the cast steel decreased by 40% in Inventive Example 2, and this factor is also considered to contribute to improvement of bending strength and durability.

In Comparative Examples 2-4 and 1-3 of the present invention, the difference in the content of the alloying elements in the inventive examples can be regarded as an error level sufficiently generated in the manufacturing process. In fact, the alloy compositions of these alloys are judged to be almost the same level, and therefore, it is considered that the effect of fine compositional difference between the embodiments except the Ca addition amount and the O content thereof is negligible.

However, as compared with Comparative Example 1 which is a general purpose material, there is a considerable difference in composition, and a large difference in bending strength can be seen as an effect thereof.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of protection of the present invention should be defined by the claims.

Claims (6)

Mo: more than 1.2% to less than 1.3%, V: not less than 0.4% to less than 0.5%, Si: not less than 0.4%, and more preferably not less than 0.1% The content of Ca is not more than 0.5% by weight, the content of Mn is not less than 0.4% and not more than 0.5%, the content of Ni is not less than 0.2% and not more than 0.25% 0.02 to 0.2%, and the balance of Fe and inevitable impurities.
The method according to claim 1,
Wherein the amount of Ca added is 0.05 to 0.1% by weight in the raw material before casting.
The method according to claim 1,
Wherein the cast steel has an O content of 120 ppm or less.
As a mold or tool,
Wherein the composition comprises, by weight%, C: more than 0.9% to less than 1.0%, Cr: more than 5.0% to less than 5.3%, Mo: more than 1.2% to less than 1.3%, V: % Or more to 0.5% or less, Mn: 0.4% or more to 0.5% or less, Ni: 0.2% or more to less than 0.25% and further containing Ca or a Ca-containing compound, 0.02 to 0.2% by weight, and the balance Fe and inevitable impurities.
5. The method of claim 4,
Wherein the amount of Ca added is 0.05 to 0.1% by weight in weight of the raw material before casting.
5. The method of claim 4,
Wherein the cast steel has an O content of 120 ppm or less.

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